A swivel of the above-mentioned type is described in WO 90/02289. In this publication, a swivel for offshore hydrocarbon production is disclosed wherein dynamic face seals are mounted in complementary stepped annular regions. In the known face seal configuration, the pressurisation effects on the swivel cause the outer ring to decrease in axial length and the inner ring to increase in axial length, hence reducing the axial space of the sealing area. The outer seal ring that contains the face seals deflects outwardly due to pressurisation hence increasing the axial sealing space. By designing the geometry in the known swivel, the effects could cancel out and the axial sealing space remains substantially at the designed dimension.
However, for the above-described face seal swivels the seal annulus gap variation is governed by several complex parameters, some of which are highly non-linear. The extension of the axial dimension of the inner swivel ring and the decrease in axial dimension of the outer swivel ring, or so-called “Poisson's effect” requires advanced Finite Element Analysis (FEA) methods. Furthermore, the radial deformations of each swivel part are not uniform along the vertical axis due to the pressurized area acting only over a limited height, as top and bottom ends of the seal annulus gap are not pressurised for zones beyond the topmost and lowermost dynamic seals. The non-uniform thickness of the swivel rings also affects the radial deformations of each swivel part. The “bowing” of each swivel part requires again advanced FEA analysis methods to arrive at matching vertical deflections of the deformed parts. Finally, varying bolt preloads and varying friction coefficients at the various bolted interfaces affect the stiffness of the different swivel parts. Also, bolt pretension and friction coefficient influence the sliding at the different bolted interfaces which in turn affects the bending (bowing) of the swivel parts.
For a top bearing face seal swivel, all the above effects mostly impact on the bottom dynamic seals that are situated furthest away from the swivel bearing. Face seal swivel fabrication also requires very high machining accuracies. Due to the large number of stacked machined rings, the cumulated fabrication tolerances can be too large, leading to unsuitable seal extrusion gaps, i.e. too large or too narrow. Current swivel designs are at the limit of best manufacturing capabilities base on the current design criteria.
Deformations in the radial and vertical direction of both piston seal and face seal swivels are non-uniform due to presence of swivel openings and external loading (piping loads). Seal grooves and seal running surfaces become oval shaped (piston seal) or are affected by surface irregularities (face seal) These so-called 3D effects lead to local deformations of the seal groove width or seal annulus gap width when the swivel rotates and are believed to be responsible for some seal failure mechanisms.
It is therefore an object of the present invention to provide a swivel that can be manufactured accurately and effectively, using relatively straight forward design tools. It is another object of the invention to provide a swivel in which 3D effects are mitigated and which reduces variations in seal annulus gap width.